Adapting to the winter

Cold depends so much on context. In the last two weeks, I’ve experienced two different winters separated by thirty degrees of latitude – Rondane National Park, in central Norway (62 °N), and now Tucson, in Arizona in the lower United States (32 °N). The experience has made me think more about how organisms (people included) respond to extreme climates.

Norway in winter is a landscape painted in snow. Above, you are seeing a photograph taken from near the summit of Formokampen – a long climb by touring skis, with a final peak only accessible on foot. Blowing snow darkens the sun, and high winds make the temperatures (already far below freezing) feel entirely intolerable.

This climate is hard on plants – most disappear under snow for the winter, and take a variety of strategies to survive. Some die back completely, leaving only seeds to regrow. Others die back to their roots and stems, relying on stored resources to begin a new round of growth. And a few, like the trees you see here, maintain leaves (needles) throughout the winter. Keeping needles and trunks intact is a hard task – ice formation inside the plant can rupture cells and prevent xylem conduits from transporting water. And heavy loads of snow can break branches. These conifers have adaptations to deal with these cold conditions, but eventually these adaptations cannot cope with even more extreme conditions, or are out-competed by plants with other strategies – as you can see at the sharply demarcated treeline beneath this mountain.

Now consider a different place, located deep in the Sonoran desert. It is a place that you might expect to be much warmer – but it is not always so. As soon as I came back to Tucson, the temperatures fell below freezing for three nights in a row. This is unusual, and a circumstance that makes the winter feel far colder than Norway. I live in a historic railroad workers’ house that hasn’t been substantially renovated since its construction – so when the temperatures outside are below freezing, they can also be below freezing inside. We have single-pane glass windows, no insulation in the walls, and not a few cracks and gaps in the walls that let cold air in. Water pipes are uninsulated and located outside the house. Yesterday I woke up to a house without water because all the pipes had frozen overnight. In Norway I stayed in a small wooden hut, but a consistently warm hut, because it was constructed with the expectation of experiencing cold winters!

This experience is similar for plants. The ecosystems of the desert floor are not well-suited to cold climates. The prickly-pear cactus (Opuntia sp.), for example, will easily lose its arms after only a day of sub-freezing temperatures. This is a loss of decades of growth and reproductive potential. Other cacti ‘melt’ from the inside out when faced with cold, and suffer similar damages. Here’s a photo of the plant in my front yard.

So this is clearly a non-adaptive strategy, unless freezing events are very rare (think of my house, built with the expectation of warm climates). If they are, then these species may actually be strong competitors the rest of the year, because they do not have to invest in costly adaptations to the cold. And this is of course the case – the desert is dominated by cacti and not pine trees, because climates are usually hot and dry. Cacti clearly can persist despite these occasional stresses, but the situation may change if these extreme events become more frequent in the future because of climate change. There is a race between climate change and species’ ability to acclimate, migrate, or evolve in response to this change. In general most species appear to be out of equilibrium with climate (see also this more recent paper), but our understanding of how much remains limited. I was just at a conference where more and more people are beginning to think about how species’ environmental niches are actually matched to their environments – it is an exciting topic, especially as climates are beginning to shift more quickly, but one that we still need to learn a lot more about. A poorly-designed house can easily be demolished or renovated – but a species, or a whole ecosystem, is a far more complex system!

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Thanks for your photos of your prickly pear! I was looking for for any others who had experienced what I did… (I live in Tempe, AZ, southeast of Phoenix). My prickly pear survived the first three nights of the freeze, but I was very shocked to see that on the morning after the fourth and last overnight freeze, several arms had broken off. Then I toured my neighborhood, and every single prickly pear suffered the same fate! This was the first time I’d seen this. In the previous years’ freezes, the below freezing temps lasted only one or two nights in a row, so there was no damage. (Except to my ficus — that’s another story…) But this year, the 4-day span showed this result for the first time for my prickly pear. I do intend to re-plant the broken off limbs and hope they take root…